1. Field of the Invention
The present invention relates to a receiving system or a mobile communications system which has a frequency stabilizing function to control a frequency stability to a predetermined value. More particularly, this invention relates to a communication system using a heterodyne receiving system, i.e., a communication system having a function of calibrating the frequency of a reference signal which is to be a reference of transmitting/receiving frequencies, which is applicable to a digital automatic telephone.
2. Description of Related Art
Prior techniques of improving the accuracy of the transmitting/receiving frequencies in a mobile communications system are disclosed in Japanese Patent Laid-Open Publication (KOKAI) Nos. 63-26020 and 63-28156. In these prior techniques, the frequency of a transmission signal is automatically settled within a predetermined range by converting a stable frequency of a received signal transmitted from a base station into a second intermediate frequency signal and then counting the frequency of the second intermediate frequency signal by means of a counter device so as to obtain a difference between the counted value and a predetermined second intermediate frequency reference value. According to the difference, an oscillation frequency control signal is issued by an operation controlling device to a VC-TCXO (Voltage Controlled Temperature Compensated Crystal Oscillator).
To obtain the frequency more accurately, the frequency of a second local oscillator is also counted by the counter device, and a frequency error of the second local oscillator is made to be corrected based upon the counted value.
In the above mentioned automatic frequency control, the relationship between the estimate accuracy of the frequency error based on the counted value and time required for counting is reciprocal. If the accuracy of an oscillator which is a time base of the counter device is sufficiently high, in principal, an error of nearly 10 Hz is estimated in a 100 ms count period.
On the other hand, in the digital automatic telephone system, microwave digital communication by time division multiplex communication has been in practical use in recent years. As a prior technique relating to a demodulator suitable for time-division multiplex communication, it is commonly known to use a demodulator with quasi-coherent demodulator which vector-demodulates with respect to a continuous phase modulating wave having a constant amplitude by use of a fixed reference carrier so as to obtain a quasi-coherent demodulation signal, as disclosed in, for example, Japanese Patent Application Laid-Open is (KOKAI) 2-46044. In this prior demodulator, a phase rotating signal caused by carrier drift owing to fading and relative deviation between the oscillation frequency of a base station and the fixed reference carrier. The frequency error of the fixed reference carrier wave is estimated by an operation method in which primary approximation is performed with respect to the accumulation of the phase rotation. This estimate processing is performed at one time with respect to burst data, and hence it is completed at high speed within the time required for operational processing started from the reception of the burst data.
In a receiver or a mobile communication system in which receiving frequency stability is required, it is essential to maintain a reference frequency in a predetermined range without controlling the system for a long time period, i.e., for five or ten years. Further, in a mobile communication, it is essential to improve the accuracy of the transmitting frequency, as further even narrower frequency bands come into use.
In the prior art techniques, there is provided a VC-OCXO to be controlled based on the receiving frequency, and the oscillation frequency of the VC-TCXO is fine-controlled on the basis of the received signal so as to obtain the desired frequency stability. Also, the frequency of a second local oscillator can be counted by switching the mode, and the oscillation frequency of the second oscillator is corrected using the counted value, which makes it possible to control the frequencies to a high accuracy of within 1 ppm.
On the other hand, however, there is a need for low power consumption in the receiver of the mobile communication system.
In the prior techniques, low power consumption cannot be achieved owing to a frequency estimator and an amplifier. The former counts high frequency which is the output from the second local oscillator, and the latter amplifies the second local oscillation frequency signal so that it can be counted by the counter.
For example, it is assumed that a first intermediate frequency is 90 MHz, and the second intermediate frequency is 455 kHz. The oscillation frequency of the second local oscillator therefore becomes 89.545 MHz, which is the threshold frequency in a CMOSIC. Taking into consideration the need for lower power consumption, this frequency exceeds the operational threshold frequency, which cannot be adequately dealt with in the CMOSIC.
Further, in the time division communication in which the operation should be performed in the burst mode, high-speed processing is required in an automatic frequency control system. The counter requires a relatively long time for counting, which makes it unsuitable for the automatic frequency control system. Therefore, it has been suggested to provide an automatic frequency control in which the estimated result is fedback to the VC-TCXO in the demodulation system having the quasi-coherent demodulator and operational device for estimating the frequency error. However, the estimate of the frequency error is executed with respect to the output of the quasi-coherent demodulator. In the mobile communication system employing the double super heterodyne system, the frequency error is estimated by the value accumulated with deviation factors such as the deviation of VC-TCXO, the frequency variations of the second local oscillator and the fixed reference carrier generator, carrier drift caused by high-speed fading, etc., and hence, it is difficult to obtain only the variation of the VC-TCXO accurately. In particular, a relatively cheap oscillator is used as the second local oscillator in the mobile telephone. When the estimated result estimated from the accumulated value is fedback directly, the accuracy of the transmission frequency relative to the base station is deteriorated. For example, when it is assumed that the first intermediate frequency is 90 MHz in the mobile telephone system in the 900 MHz band, and the frequency stability of the second local oscillator is 10 ppm, 900 Hz frequency error is obtained. When this is converted as the frequency error of 900 MHz, the deterioration is generated by 1 ppm. Taking into consideration the high-accurate of frequencies in recent years, this deteriorated value is not negligible.
To improve the accuracy of the transmission frequency, there may be provided an oscillator having a high performance as the second local oscillator, which is however not preferable in cost.
Further, it is difficult to make the counter into the LSI circuit owing to the high frequency to be counted, which reduces the level of minimization of the system. This is a serious problem particularly to a portable communication system required to be reduced in size, because it needs to be operated with a low voltage in order to reduce the number of cells.
Further, the second local oscillator has been included in the LSI circuit in recent years. To pick up the output of the second local oscillator securely, it is necessary to provide an amplifying circuit which gives no influence to the operation of the LSI. However, this leads to an increase in the number of circuits in the system, and is therefore not suitable for the portable type.